CHAPTER VII.

PRESSURE.

_A._ =Lines of Equal Pressure: Isobars.=——One of the most important weather elements is the _pressure_ of the atmosphere. This has already been briefly discussed in the sections on the mercurial barometer (Chapter II). It was there learned that atmospheric pressure is measured by the number of inches of mercury which the weight of the air will hold up in the glass tube of the barometer. Our sensation of heat or cold gives us some general idea as to the air temperature. We can tell the wind direction when we know the points of the compass, and can roughly estimate its velocity. No instrumental aid is necessary to enable us to decide whether a day is clear, fair or cloudy, or whether it is raining or snowing. Unlike the temperature, the wind, or the weather, the pressure cannot be determined by our own senses without instrumental aid. The next weather element that we shall study is pressure.

Proceed as in the case of the thermometer readings. Enter upon a blank map the barometer readings for the different stations given in the third column of the table in Chapter VIII. When this has been done you have before you the actual pressure distribution over the United States at 7 A.M., on the first day of the series. Describe the distribution of pressure in general terms. Where is the pressure highest? Where lowest? What are the highest and the lowest readings of the barometer noted on the map? What is the difference (in inches and hundredths) between these readings?

Draw lines of equal pressure, following the same principles as were adopted in the case of the isotherms. The latter were drawn for every even 10° of temperature. The former are to be drawn for every even .10 inch of pressure. Every station which has a barometer reading of an even .10 inch will be passed through by some line of equal pressure. Philadelphia, Pa., with 29.90 must be passed through by the 29.90 line; Wilmington, N. C., with 30.00, must have the 30.00 line passing through it, etc. Chicago, with 30.17 inches, must lie between the lines of 30.10 and 30.20 inches, and nearer the latter than the former. Denver, Col., with 30.35 inches, must lie midway between the 30.30 and 30.40 lines (Fig. 32).

Lines of equal pressure are called _isobars_, a word derived from two Greek words meaning _equal pressure_.

Describe the distribution of pressure as shown by the arrangement of the isobars. Note the differences in form between the isotherms and the isobars. The words _high_ and _low_ are printed on weather maps to mark the regions where the pressure is highest and lowest.

Draw isobars for the other days, using the barometer readings given in the table in Chapter VIII. Figs. 33-38 show the arrangement of the isobars on these days.

The pressure charts may be colored, as indicated by the shading in these figures, in order to bring out more clearly the distribution of pressure, according to the same general scheme as that adopted in the temperature charts. Color _brown_ all parts of your six isobaric charts over which the pressures are below 29.50 inches; color _red_ all parts with pressure above 30.00 inches. Use a _faint shade of brown_ for pressures between 29.50 inches and 29.00 inches, and a _darker shade_ for pressures below 29.00 inches. In the case of pressures over 30.00 inches, use a _pale red_ for pressures between 30.00 and 30.50 inches, and a _darker shade of red_ for pressures above 30.50 inches. By means of these colors the pressure distribution will stand out very clearly. The scheme of color and shading may, of course, be varied to suit the individual fancy.

Study the isobaric chart of each day of the series by itself at first. Describe the pressure distribution on each chart.

Then compare the successive charts. Note what changes have taken place in the interval between each chart and the one preceding; where the pressures have risen; where they have fallen, and where they have remained stationary. Write a brief account of the facts of pressure change illustrated on the whole series of six charts.

Compare the charts of temperature and of pressure, first individually, then collectively. What relations do you discover between temperature distribution and pressure distribution on the isothermal and the isobaric charts for the same day? What relations can you make out between the changes in temperature and pressure distribution on successive days? On the whole series of maps? Write out the results of your study concisely and clearly.

Compare the wind charts and the pressure charts for the six days. Is there any relation between the direction and velocity of the winds and the pressure? Observe carefully the changes in the winds from day to day on these charts, and the changes in pressure distribution. Formulate and write out a brief general statement of all the relations that you have discovered.

=Mean Annual and Mean Monthly Isobaric Charts.=——We have thus far been studying isobaric charts based on barometer readings made at a single moment of time. Just as there are mean annual and mean monthly isothermal charts, based on the mean annual and mean monthly temperatures, so there are mean annual and mean monthly isobaric charts for the different countries and for the whole world, based on the mean annual and mean monthly pressures. The mean annual and mean monthly isobaric charts of the world show the presence of great oval areas of low and high pressure covering a whole continent, or a whole ocean, and keeping about the same position for months at a time. Thus, on the isobaric chart showing the mean pressure over the world in January, there are seen immense areas of high pressure (anticyclones) over the two great continental masses of the Northern Hemisphere. These anticyclonic areas, although vastly greater in extent than the small ones seen on the weather maps of the United States, have the same system of spirally _outflowing_ winds. Over the northeastern portion of the North Pacific and the North Atlantic, in January, are seen immense areas of low pressure (cyclones) with spirally _inflowing_ winds. In July the northern continents are covered by cyclonic areas, and the central portion of the northern oceans by anticyclonic areas.

_B._ =Direction and Rate of Pressure Decrease: Pressure Gradient.=——In